The present invention relates to polyolefin compositions for electrical insulation having high electrical breakdown strength.
Conventionally various attempts have been made to improve the electrical breakdown strength of polyolefin by adding additives to polyolefin. Some of these attempts have matured to improvements as specifically disclosed in U.S. Pat. Nos. 3,542,648 and 3,126,358, British Patent No. 1,202,910 and Japanese Patent Publication No. 19663/1970, etc.
However, the polyolefin compositions proposed have the drawback that when subjected to exposure at elevated temperatures or subjected to heat cycle, the compositions loss their initial breakdown strength. Moreover the polyolefin compositions disclosed in prior arts may be not suitable as insulating materials for use at an extra-high voltage, such as 275 kV.
Our researches have revealed that a polyolefin composition prepared by adding a small amount of ferrocene compound and an 8-substituted quinoline compound to polyolefin exhibits improved electrical breakdown strength, and is capable of retaining excellent voltage endurance even when subjected to heating or heat cycle. Further, incorporating of a siloxane oligomer together with above additives, the resultant composition shows more improved electrical breakdown strength and also exhibits superior voltage endurance over a long period of emmersion in water.